In positive-displacement compressors, capacity control may be obtained by both speed modulation and suction throttling to reduce the volume of vapor or gas drawn into a compressor. Positive displacement compressors include, for example, reciprocating compressors, rotary compressors, scroll compressors and screw compressors. Screw compressors, also known as helical lobe rotary compressors, are well-known in the air compressor refrigeration, water chiller and natural gas processing industries.
Reciprocating compressors utilize a movable piston in a cylinder. The piston is attached to a connecting rod which is attached to a crank. An electric motor drives the crank which causes the piston to reciprocate within the cylinder, increasing and decreasing the volume within the cylinder. Fluid is introduced into the cylinder through a valve when the piston is at the top of its stroke. The fluid is compressed and removed from the cylinder through a valve when the piston is at the bottom of the its stroke.
Scroll compressors generate a series of crescent-shaped pockets between two scrolls, the crescent-shaped pockets receiving fluid for compression. Typically, one scroll is fixed and the other orbits around the fixed scroll. As the motion occurs, the pockets between the two forms are slowly pushed to the center of the two scrolls. This reduces the fluid volume.
Rotary compressors are of two general types: stationary blade and rotating blade compressors. The blades or vanes on a rotating blade rotary compressor rotate with the shaft within a cylindrical housing. In a stationary blade compressor, the stationary blade has a blade that remains stationary and is part of the housing assembly, while a cylinder rotates within the housing assembly, via a roller on an eccentric shaft within the cylinder. In both types, the blade provides a continuous seal for the fluid. Low pressure fluid from a suction line is drawn into an opening. The fluid fills the space behind the blade as it revolves. The trapped fluid in the vapor space ahead of the blade is compressed until it can be pushed into the compressor exhaust.
Screw compressors generally include two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side walls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to the ground. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing.
Common to each type of compressor is an inlet, an outlet and a working chamber. A compressor inlet is sometimes also referred to as the “suction” or “low pressure side,” while the discharge is referred to as the “outlet” or “high pressure side.” Refrigerant gas, after passing through the inlet, is compressed to a higher pressure in the working chamber. A mechanical means acts on the refrigerant gas to compress it from a first pressure to a second chamber. The mechanical means for compressing the refrigerant gas differs among the various positive displacement compressors. The compressed refrigerant gas then passes from the compressor through an outlet or discharge port to the remainder of the refrigeration system.
Screw compressor rotors intermesh with one another and rotate in opposite directions in synchronization within a housing. The rotors operate to sweep a gas through the housing from an intake manifold at one end of the housing to an output manifold at the other end of the housing. Commercially available screw compressors most commonly include threaded shafts or helical rotors having four lobes, however, others have been designed to have five or more lobes; however, it may be possible to use rotors having 2-5 lobes. The rotor shafts are typically supported at the end walls of the casing by lubricated bearings.
Capacity control for such compressors can provide continuous modulation from 100% capacity to less than 10% capacity, good part-load efficiency, unloaded starting, and unchanged reliability. In a refrigeration system, capacity also can be regulated based upon a temperature set point for the space being cooled. In other systems where the compressor is processing gas, capacity may be regulated to fully load the torque generator or prime mover (turbine or engine drive) for the compressor. However, all of the currently available methods are expensive and add to the initial cost of investment in the equipment.
In chiller applications where economy is desired both in the initial cost of the system and in operation of the system, a variable volume ratio application is desired. In a screw compressor, the volume, or compression ratio Vi, is the ratio of the volume of a groove at the start of compression to the volume of the same groove when the discharge port begins to open. Hence, the volume ratio in a screw compressor is determined by the size and shape of the discharge port.
For maximum efficiency, the pressure generated within the grooves during compression should exactly equal the pressure in the discharge line when the volume begins to open to it. If this is not the case, either over-compression or under-compression occurs, both resulting in internal losses in efficiency. Such losses in efficiency increase power consumption and/or noise, while reducing efficiency.
If the operating conditions of the system seldom change, it is possible to specify a fixed-volume ratio compressor that will provide good efficiency. But since over-compression can cause damage to a compressor, compressors are designed to limit over-compression, so they do not frequently operate in an over-compression mode. Compressors designed to limit over-compression are often designed to run at a maximum or substantially maximum compression under the most severe operating conditions. When not under the most severe operating conditions, the fixed-volume ratio compressor designed to limit over-compression will run in under-compression mode, which results in at least reduced efficiency.
What is needed is a system that permits adjustments to the volume ratio depending on the conditions that the compressor experiences. This will allow the compressor discharge volume to be adjusted to change the discharge volume, and hence the volume ratio, as operating conditions change resulting in a change in refrigeration demand, allowing the compressor to operate at increased an improved efficiency.